Elastin is an amorphous protein present in the elastic fibers present in such tissues as blood vessels, skin, tendons, ligaments, and lungs. Elastic fibers are also present in periodontal micro-ligaments and those surrounding hair follicles in the skin. Unlike other fibrous tissues like collagen, elastin is unique in that it may be stretched to over 150 percent of its original length but it can rapidly return to its original size and shape. This property of elastin provides tissues that incorporate it, the required ability to resume their original form after stretching due to blood flow, breathing, or bending. Like collagen protein, elastin contains about 30% glycine amino acid residues and is rich in proline. Elastin differs from collagen in that it contains very little hydroxyproline and no hydroxylysine. It is particularly rich of alanine and also contains two unique amino acids isodesmosine and desmosine.
The extracellular matrix (ECM) of the skin and other connective tissues comprises of numerous glycosaminoglycans, protoglycans, fibronectin, laminin and collagen and elastic fibers. The resiliency of skin is maintained by elastic fibers. These ECM components are organized into a networks of rope-like structures and composed of two major components: an amorphous core, consisting of unique polymeric protein, elastin which makes up the bulk (>90%) of the fiber; and the 10-12-nm microfibrils made up of several distinct glycoproteins, e.g., fibrillins, fibulins and microfibril-associated glycoproteins (MAGPs). In arterial walls elastin and microfibrils are organized in the form of multiple concentrated membranes, that are responsible for arterial resiliency. Elastic fiber formation (elastogenesis) is a complex process involving several intracellular and extracellular events. Cells (fibroblasts, endothelial cells, chondroblasts or vascular smooth muscle cells) must first synthesize and secrete numerous glycoproteins to form a microfibrillilar scaffold. In these cells tropoelastin is synthesized by ribosomes in the rough endoplasmatic reticulum and transported through the Golgi apparatus and secretory vesicles. Tropoelastin, the soluble precursor peptide of elastin, with a molecular weight in the range of 70-75 kDa, is properly assembled and covalently cross-linked to form the unique composite amino acids called desmosines and isodesmosines by lysyl oxidase into a resilient polymer, insoluble elastin. Production of elastin reaches its highest levels in the third trimester of the fetal life and steadily decreases during early postnatal development. In undisturbed tissues elastic fibers may last over the entire human lifespan. Mature (insoluble) elastin is metabolically inert and remains the most durable element of extracellular matrix, that may last for the lifetime in the undisturbed tissues.
The net deposition of elastin appears to be controlled on both the transcriptional level (tropoelastin mRNA message expression) and-post-transcriptional level (tropoelastin message stability). There are also several other post-transcriptional events, which control secretion of tropoelastin monomers and their proper extracellular assembly and regulate the cross-linking of tropoelastin into the polymeric “insoluble” elastin, the most durable element of the extracellular matrix.
In various tissue or biological functions, non-elastic collagen fibers may be interwoven with the elastin to limit stretching of the elastin and prevent tearing of elastin comprising tissue. However, in contrast to life-long-lasting elastin, collagens which half life differs from months to years, have to be periodically replaced.
Different components of the extracellular matrix have been solubilized and previously incorporated into cosmetic compositions. Because normally cross-linked and highly hydrophobic elastin is insoluble in water, organic solvents, and physiological fluids, more radical chemical and enzymatic methods have to be used to cleave insoluble elastin protein to form smaller peptide fragments, that may be eventually used for cosmetic formulations.
The human skin consists of two layers; a superficial layer called the epidermis which is epithelial tissue and a deeper layer called the dermis that is primarily connective tissue. These two layers are bound together to form skin which varies in thickness from less than about 0.5 mm, to 3 or even 4 millimeters. The connective tissue found in skin is essentially an intricate meshwork of interacting, extracellular molecules that constitute the so-called “extracellular matrix” (ECM). Particular components of the ECM (proteoglycans and proteins) are secreted by local fibroblasts and eventually form the dermal meshwork that not only mechanically support the cells and blood vessels, but also modulate the proper hydration of the skin. Exposure of the skin to ultraviolet and visible light from the sun, wind, and certain chemicals may cause loss of moisture and structural damage of the existing ECM, that eventually lead to lack of elasticity local collapses (wrinkles) of the dermal tissue supporting epidermal layers. Severe loss of elasticity occurs in response to degradation of the elastic fibers and the fact that in contrast to other ECM components they can not be quickly replaced by local “unstimulated” cells. These clinically observed symptoms, characterized by a lose of normally assembled elastic fibers and accumulation of amorphous and often calcified “clumps” in the dermoepidermal junction and papillary dermis is commonly referred to as solar elastosis.
Until recently, elastin, the major component of elastic fibers, was thought to have primarily a mechanical role in providing tissue resiliency. This view was challenged by results of in vitro studies indicating that soluble fragments of tropoelastin and elastin degradation products may bind to the cell surface Elastin Binding Protein (EBP) and stimulate proliferation and migration of human skin fibroblasts, lymphoblasts, smooth muscle cells and cancer cells.
In addition to primary elastinopathies that have been directly linked to alterations in the elastin gene (supravalvular aortic stenosis (SVAS), Williams-Beuren syndrome (WBS) and cutis laxa), a number of secondary elastinopathies have been described, caused by functional imbalance of other structural and auxiliary factors regulating elastic fiber deposition (Marfan disease, GM-1-gangliosidosis, Morquio B, Hurler disease, Costello syndrome, Ehlers Danlos syndrome, pseudoxanthoma elasticum (PXE)). A lack of elastin or genetic abnormalities affecting elastic fibers in skin, as evidenced in Costello Syndrome, Cutis Laxa and Pseudoxanthoma Elasticum respectively, lead to premature aging most noticeably characterized by wrinkling and folding of the skin in children (pre-teenage) suffering from these illnesses. Given that these conditions only affect elastic fibers in skin, it is highly probable that development of wrinkles in aged skin is due to damage to or loss of elastic fibers in skin. Unfortunately, dermal fibroblasts lose their ability to make elastin (the major component of elastic fibers) by the end of puberty. Hence, adult dermal fibroblasts cannot repair or replace damaged elastic fibers in skin later in life, leading to an essentially irreversible formation of wrinkles.
Diffuse elastic fiber defects, resembling those reported in inherited PXE have also been detected in patients with β-thalassaemia and sickle cell anemia, and in other hemolytic anemias. Genetic basis for these diseases cannot be directly linked to any structural or regulatory components involved in elastic fiber production. However, it has been suggested that the accumulation of iron in these patients, resulting from hemolysis, increased iron absorption, and multiple blood transfusions may lead to acquired elastic tissue defects.
Iron is a physiologically essential nutritional element for all life forms. It plays critical roles in electron transport and cellular respiration, oxygen transport by hemoglobin, cell proliferation and differentiation. It has been shown that modulating intracellular iron levels may also affect expression of numerous genes that are not directly involved in iron metabolism, such as protein kinase C-β (PKC-β), an important component of intracellular signaling pathways, or those encoding extracellular matrix (ECM) components. It has been demonstrated that dietary iron overload in rats resulted in an increase in the steady-state level of pro-α2(I)-collagen in hepatocytes, and that 50 μM iron treatment stimulated collagen gene expression in cultured stromal hepatic cells, by inducing the synthesis and binding of Sp1 and Sp3 transcription factors to two regulatory elements located in the collagen α1 (I) promoter region. On the other hand, iron loading in cultured cardiac myocytes and fibroblasts decreased the expression of TGF-β, biglycan, and collagen type I mRNA, while it facilitated the expression of decorin mRNA. Interestingly, iron deprivation exerted a similar effect, suggesting that the expression of these genes involved in extracellular matrix production is regulated by certain iron-dependent mechanisms.
The molecular basis of iron-dependent mechanism(s) regulating the expression of ECM encoding genes are not well understood. Since raising levels of iron may overwhelm the iron-binding capacity of transferrin, resulting in the appearance of non-transferrin bound iron (NTBI), which is capable to catalyze the formation of the hydroxyl radicals (through the Fenton and Haber-Weiss reactions), it has been suggested that iron-dependent induction of reactive oxygen species (ROS) may modulate the transcription of these genes. The possibility of iron-dependent oxidative damage to elastic fibers has also been suggested, but not proven.
Manganese is an essential trace nutrient in all forms of life. The classes of enzymes that have manganese cofactors are very broad and include such classes as oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases, lectins, and integrins. The best known manganese containing polypeptides may be arginase, manganese containing superoxide dismuates, and the diptheria toxin.
It has been found that certain minerals and therapeutic compositions containing the same can increase synthesis of elastin. In particular, such minerals can stimulate proliferation of (normally dormant) fibroblasts derived from adult human skin and induce synthesis of elastin and collagen in human fibroblasts and smooth muscle cells. These minerals may also induce synthesis of tropoelastin, deposition of insoluble elastin, and increase elastin mRNA levels. Stimulation of cellular rejuvenation may be enhanced by administering a therapeutic composition comprising divalent manganese, trivalent iron and salts thereof.